參數(shù)資料
型號: MPC5607BCMG4
廠商: FREESCALE SEMICONDUCTOR INC
元件分類: 微控制器/微處理器
英文描述: FLASH, 48 MHz, MICROCONTROLLER, PBGA208
封裝: 17 X 17 MM, 1 MM PITCH, MO-151 AAF-1, BGA-208
文件頁數(shù): 78/106頁
文件大?。?/td> 667K
代理商: MPC5607BCMG4
Electrical characteristics
MPC5607B Microcontroller Data Sheet, Rev. 5
Freescale Semiconductor
73
Figure 20. Spectral representation of input signal
Calling f0 the bandwidth of the source signal (and as a consequence the cut-off frequency of the antialiasing filter, fF), according
to the Nyquist theorem the conversion rate fC must be at least 2f0; it means that the constant time of the filter is greater than or
at least equal to twice the conversion period (TC). Again the conversion period TC is longer than the sampling time TS, which
is just a portion of it, even when fixed channel continuous conversion mode is selected (fastest conversion rate at a specific
channel): in conclusion it is evident that the time constant of the filter RFCF is definitively much higher than the sampling time
TS, so the charge level on CS cannot be modified by the analog signal source during the time in which the sampling switch is
closed.
The considerations above lead to impose new constraints on the external circuit, to reduce the accuracy error due to the voltage
drop on CS; from the two charge balance equations above, it is simple to derive Equation 11 between the ideal and real sampled
voltage on CS:
Eqn. 11
From this formula, in the worst case (when VA is maximum, that is for instance 5 V), assuming to accept a maximum error of
half a count, a constraint is evident on CF value:
ADC_0 (10-bit)
Eqn. 12
ADC_1 (12-bit)
Eqn. 13
f0
f
Analog source bandwidth (VA)
f0
f
Sampled signal spectrum (fC = Conversion rate)
fC
f
Anti-aliasing filter (fF = RC filter pole)
fF
2 f0 < fC (Nyquist)
fF = f0 (Anti-aliasing filtering condition)
TC < 2 RFCF (Conversion rate vs. filter pole)
Noise
V
A
V
A2
------------
C
P1
C
P2
+C
F
+
C
P1
C
P2
+C
F
C
S
++
--------------------------------------------------------
=
C
F
2048 C
S
C
F
8192
C
S
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